The invention concerns a transducer structure operating with acoustic waves, particularly for a surface wave filter (called an OFW or also a SAW filter) or an S-BAR filter (bulk acoustic wave resonator).
In filters operating with acoustic waves (particularly in SAW filters) in particular transducer structures operating with acoustic waves (for example, SAW resonators) are used as impedance elements. Such a resonator is constructed on the surface of a piezoelectric substrate from metallic electrode structures, and comprises an inter-digital transducer with at least two connections that is normally arranged between two reflectors. Known resonators comprise inter-digital transducers that are characterized by a uniform finger period and finger width over the entire transducer. Each resonator thereby exhibits what is known as a resonance frequency and an anti-resonance frequency. The frequency position and the intensity of resonance and anti-resonance can be influenced via variation of the apertures, the finger count and the finger period. The frequency separation between resonance frequency and anti-resonance frequency, as well as its form, remains the same.
In the reactance filter, the resonators are used as impedance elements and switched to an arrangement resembling a ladder (i.e., a ladder type). In addition, resonators are arranged in one serial branch and at least one (preferably, however, a plurality of) parallel branche(s). The resonance frequency of a resonator in the serial branch is set such that it approximately corresponds to the anti-resonance frequency of a resonator in the parallel branch. More complex filters, with a plurality of parallel branches and serially arranged resonators between them, can be constructed from a plurality of basic components (that respectively comprise a parallel and a serial resonator). The interaction of the resonances of the individual resonators generates a desired band-pass behavior of the filter. In addition, the resonance frequencies of the individual resonators, as well as the intensity of the resonances, are suitably set. For this purpose, finger periods, finger counts, and apertures of the individual resonators are the known degrees of freedom.
An ideal filter exhibits a good electrical conformance, a good damping behavior in the filter attenuation band, and as little insertion loss as possible in the transmission range. However, its disadvantage is that these characteristics can mostly not be simultaneously optimized, such that only one suitable combination of characteristics can always be achieved, but not one filter optimal in all characteristics. Particularly in broadband filters that exhibit a relative bandwidth of more than 2%, or in filters that are constructed on substrates with lower electro-acoustic coupling (for example, on LiTaO3 in connection with smaller layer thickness or on quartz), an optimization attempt can only result in non-optimal filters with unsatisfactory characteristics.